System for forming a reaction product such as calcium silicate

ABSTRACT

A system for forming a reaction product such as calcium silicate comprises an autoclave for receiving the reaction constituents and for reacting these constituents to form a reaction product, a holding vessel for receiving the reaction product and a flow passage connecting the autoclave to the holding vessel and for allowing the passage of the reaction product from the autoclave to the holding vessel. The flow passage includes a heat exchanger for transferring heat from the reaction product during its passage through the flow passage to another medium. The system includes an electronic control system for maintaining the pressure in the holding vessel a controlled amount beneath the pressure in the autoclave during the transfer of the reaction product from the autoclave to the holding vessel. This minimizes the structural damage to the reaction product during its transfer to the holding vessel. The holding vessel can, if desired, be used to mix additional material into the reaction product.

This is a division of application Ser. No. 12,886 filed Feb. 18, 1979now U.S. Pat. No. 4,238,240.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of forming a reaction product such ascalcium silicate, structure employed in the method for forming thisreaction product, and the resulting reaction product.

2. Prior Art

Shaped calcium silicate insulation is widely used, particularly forapplications involving temperatures above 800° fahrenheit. A variety ofprocesses for forming calcium silicate insulation products are known.For example, U.S. Pat. Nos. 3,988,419, 2,699,097, 2,904,444, and3,001,882 disclose methods for forming calcium silicate insulation. Asdisclosed in the '882 patent, the calcium silicate insulation typicallyis composed of crystals of synthetic tobermorite and/or xonotliteprepared by the induration of aqueous lime-silica slurries in which themolar ratio of lime-silica falls in the range of about 0.65:1 to 1:1 andthe water-to-solids ratio falls between about 0.75:1 to about 9.0:1.Typically, in the preparation of low density insulation (i.e., densitiesranging from about 5 to 15 lbs. per cubic foot) asbestos fibers havebeen added as a reinforcing material to the slurry. A description ofcertain prior art techniques for producing molded materials of calciumsilicate is described in U.S. Pat. No. 3,679,446 on an application ofKubo. Kubo states that it is difficult to obtain calcium silicateinsulation products with uniform properties and satisfactory mechanicalstrength unless the induration reaction is conducted for a long periodof time. Kubo further states that the calcium silicates typicallyproduced cannot satisfactorily withstand high temperatures with theresult that a calcium silicate product composed mainly of tobermoritecrystals is liable to decrease in mechanical strength markedly at 650°C. or thereabout and to disintegrate or break down at over 700° C. andthat a product composed mainly of xonotlite crystals tends to decreasein mechanical strength markedly at a temperature higher than about 1000°C. Kubo discloses a method of forming calcium silicate crystals whereinat least a given percent by weight of the calcium silicate crystals hasformed therein "numerous small agglomerates of a diameter of ten to 150microns by being three dimensionally interlocked with one another," saidagglomerates being dispersed in water "in substantially globular form."Kubo also discloses the use of reinforcing fibers formed predominantlyof pulp fiber.

Hoopes and Weber disclose in U.S. Pat. No. 3,736,163 the formation ofcalcium silicate insulating material having densities on the order of 10to 15 lbs. per cubic foot wherein asbestos reinforcing fibers arereplaced by a reinforcing fiber comprising from about three percent tofifteen percent of the weight of the calcium silicate material andconsisting of nodulated mineral wool and cellulosic fiber, at leastabout twenty-five percent of the fiber being nodulated mineral wool.

In the manufacture of calcium silicate products in accordance with theprior art, the reaction constituents (typically calcium oxide (CaO) orhydrated calcium oxide Ca(OH)₂) are mixed with a siliceous material,such as sand, in water to form a slurry. This mixture is heated in anautoclave to form a variety of crystalline forms of calcium silicatedepending upon the temperature, pressure, length of reaction time andwater concentration used. Fibrous materials such as asbestos, which arenot adversely affected by the reaction conditions, may be incorporatedinto the mixture prior to processing. The reaction product of thisprocessing is generally an aqueous slurry of hydrated calcium silicatecrystals intermixed with desired fibrous components. This slurry is thencast into molds and dried, usually by heating, to form the desiredfinished shaped objects.

As discussed in the Zettel U.S. Pat. No. 3,816,149, this processing toform the crystalline materials in the slurry is time consuming andrequires large and expensive pieces of processing equipment. Thus theprior art has attempted to improve the process conditions under whichcrystallization takes place and to shorten the time required to producea finished hydrate. Decreasing the time to process the slurry throughcrystallization and the return to ambient conditions results in moreefficient and economical utilization of the equipment and an increasedoutput of finished product.

In the prior art typically the slurry was cooled in the autoclave uponcompletion of the crystallization. The pressure was then reduced withinthe autoclave while the slurry was cooled to ambient. The prior artrecognized that allowing the steam pressure in the autoclave to bereduced by cooling was both slow (because of the long time required totransfer heat from the slurry through the autoclave walls to theambient) and inefficient (because of the waste of the heat sotransferred). To speed up the process, the high pressure steam in theautoclave was vented to the atmosphere. Because at atmospheric pressurethe crystalline reaction of the components in the slurry proceeds at atemperature well above the boiling point of water, the venting of steamto the atmosphere caused the hot aqueous slurry to boil violently. Thisfractured many of the newly formed crystals thus defeating the purposeof the careful prior crystallization. Moreover, the venting of steamwasted energy. As disclosed in U.S. Pat. No. 3,816,149, Zettel attemptedto overcome these problems by hydrothermally reacting a highlyconcentrated aqueous slurry of a source of calcium oxide and a siliceousmaterial in the presence of saturated steam under elevated pressure in apressure vessel to form crystalline calcium silicate. Following theformation of the crystalline calcium silicate hydrate, the steam inputwas halted and low temperature water was gradually added to the reactionmixture within the pressure vessel until sufficient water was added todilute the crystalline slurry to the desired concentration forsubsequent molding operations. Incoming water condensed the steam in thepressure vessel simultaneously reducing its pressure and cooling thecrystal-containing slurry. The gradual cooling nd depressurization wasdescribed by Zettel as effectively eliminating the disruption of thecrystal structure. Zettel also reduced substantially the time requiredto raise the reaction mixture from ambient to the condition for thecrystallization reaction to take place by reducing the amount of waterpresent which must be heated compared to the then prior art processes.

Calcium silicate insulation produced by the prior art still leaves muchto be desired in the way of strength, high temperature insulationcapability, predictability of characteristics, machineability, anddimensional consistency. In addition, the manufacturing process for thismaterial still wastes considerable energy by heating and cooling thereaction slurry in the autoclave. This process also increases the costof forming insulation by tying up the equipment for a long period oftime per batch of calcium silicate formed. Finally, during the transferof the calcium silicate reaction product from the autoclave to a holdingtank for the next stage in the operation, the crystalline structure ofthe reaction product fractures or is otherwise changed.

SUMMARY OF THE INVENTION

This invention overcomes many of the problems of the prior art methodsof forming a reaction product such as calcium silicate in an autoclave.In accordance with this invention, reaction constituents such as silicondioxide and calcium oxide, are mixed with water in the autoclave andthen heated to a selected temperature for a selected time. Heatincreases the pressure in the autoclave to a selected level and thereaction takes place over a desired period of time. At the end of thistime, the contents of the autoclave are transferred from the autoclaveto another vessel connected by a flow passage to the autoclave. Thetransfer is carried out in one embodiment by maintaining the pressure inthe autoclave a selected and substantially fixed amount above thepressure in the receiving vessel during the transfer operation.

In one embodiment of this invention, the contents of the autoclave aretransferred to the other vessel through a heat exchanger thereby totransfer a portion of the heat in these contents to another materialthus to decrease the net amount of energy used in forming the reactionproducts. This other material is, in one embodiment, water which is areaction constituent to be used in the next batch of material to beplaced in the autoclave.

The flow rate of the material from the autoclave through the flowpassage is selected to ensure that the reaction products are not alteredas a result of the flow process. This condition is met by a flow whichis at least partly laminar and thus the Reynolds number of the reactionproducts through the flow passage is preferably less than the value atwhich the flow changes from laminar to turbulent. Satisfactory resultshave been obtained, however, when this flow is somewhat turbulent, butcompletely turbulent flow has been found to be detrimental to thequality of the final molded product.

The transfer of heat from the reaction products to the other materialduring the transfer of the reaction product from the autoclave to theholding vessel, is done at such a rate as to stabilize the reactionproducts in a desired form. When the reaction products comprise calciumsilicate formed from a stochiometric mixture of calcium oxide (CaO orCa(OH)₂) and silicon dioxide (SiO₂), the reaction products comprisecrystals of xanotlite, tobermorite and other less well-defined hydratedforms of calcium silicate. Preferably, however, the reaction product isxonotlite. The heat transfer from the xonotlite to, for example, water,stabilizes the xonotlite crystals and helps prevent the fracture orrupture of these crystals as a result of the shearing and turbulence ofthe flow process.

In the vessel, fibrous materials such as mineral wool and wood pulp areadded to the reaction products to give strength to the finished product.Typically, the mineral wool is added first followed by wood pulp. Boththe mineral wool and the wood pulp are mixed substantially uniformlythrough the reaction products.

The reaction products from the vessel mixed with the fibrous materialare then allowed to flow from the vessel to a dewatering station,typically a rotating drum upon which a vacuum is pulled. From therotating drum the dewatered reaction products are then sent to a moldwhere a vacuum is additionally pulled and they are pressed to removeadditional moisture and to form the reaction product into a desiredshape such as a rectangular slab or a curved section. The moldedproducts are then dried, preferably by infrared radiation, to removeadditional moisture. The final product has a density of about 200kilograms per cubic meter up to about 400 kilograms per cubicmeter andhas been found particularly suitable for high temperature applications,i.e., temperatures above 1500° F. The molded product exhibits improvedstrength compared to prior art products and is machineable, if desired,thereby increasing the utility of the product. However, the finalproduct preferably is molded to size in order to avoid any structuralweaknesses that might be caused by machining and to avoid waste. Theproduct also exhibits improved dimensional stability.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically an arrangement of the several pieces ofequipment used to practice the process of forming reaction products inaccordance with this invention;

FIG. 2 illustrates an electronic circuit used to control the pressure ofthe autoclave 10 (FIG. 1) relative to the pressure in the vessel 12(FIG. 1) in accordance with this invention; and

FIG. 3 comprises a table illustrating the operation of the structure ofFIG. 1 and the circuit of FIG. 2.

DETAILED DESCRIPTION

This invention will be described in relation to the reaction of calciumoxide or calcium hydroxide and silicon dioxide to form calcium silicate.However, certain principles of this invention can also be used inconjunction with other reactions requiring equipment of the same type asthat described herein, particularly where it is desirable to insure thatthe reaction products are not fractured or altered as a result of thetransfer of the products from the autoclave to another vessel.Furthermore, certain principles of this invention will also beapplicable in other reaction processes where it is desired to reduce thenet energy consumed in the process.

Turning to FIG. 1, the equipment used to illustrate the process inaccordance with this invention is shown schematically. Autoclave 10, ofa type well known in the art, comprises a pressure vessel capable ofwithstanding pressures at least in excess of from 15 to 25 bars absolutepressure. To form calcium silicate, calcium hydroxide (Ca(OH)₂) andpowdered quartz (SiO₂) are placed in the autoclave and mixed with waterso as to form less than about seven percent by weight of the totalweight of water in the autoclave. Nothing else is added to theautoclave. Preferably calcium hydroxide in powder form is used ratherthan calcium oxide (quick lime) because powdered calcium hydroxide isfree flowing and easily handled while being weighed and otherwiseprocessed. On the other hand, calcium oxide is dangerous and difficultto handle. After the reaction constituents have been added to theautoclave 10, the autoclave is heated to between 176° C. and 240° C. andpreferably to 197.4° C. at about 15 bars absolute pressure. (In thisspecification all pressures, unless otherwise indicated or obvious fromtheir context, are intended to be absolute pressures.) At this pressureand temperature the steam in the autoclave is saturated. In accordancewith this invention, the calcium oxide (or calcium hydroxide) andsilicon dioxide are preferably added in the stochiometric ratio so as tohave a ratio of one mole of calcium oxide for each mole of silicondioxide. However, if desired, the lime/silica ratio can be varied from0.65 to 1.0 to 1.2 to 1.0.

Prior to the addition of the calcium hydroxide and silicon dioxide tothe autoclave 10, but after the addition of water, agitator 10c isturned on and is left on throughout the reaction to thoroughly mix thereaction products and to insure substantially uniform results.

The result of the reaction within the autoclave is the formationpreferably of crystals of xonotlite. At the selected temperature andpressure (197.4° C. and 15 bar) the reaction typically must continue forin excess of about 55 minutes and preferably one hour and fifteenminutes but not in excess of one hour and forty-five minutes. Continuingthe reaction time beyond 55 minutes (which is the approximate timeneeded to yield xonotlite crystals under the selected conditions)increases the size of the xonotlite crystals. Since, however, by onehour and fifteen minutes the crystals are already adequate for use ininsulation, continuing the reaction beyond one hour and fifteen minutesmerely wastes energy. This one hour and fifteen minutes is measured fromthe time after the reaction constituents are in autoclave 10 andmanometer 10a has reached 15 bar absolute due to direct steam injectioninto autoclave 10.

The pressure in the autoclave 10 is controlled to within 15 bar ±1% barusing a conventional regulator of a type commercially available such asfrom Foxboro Instruments, for example. Because the steam in theautoclave 10 is saturated, maintaining the pressure at 15 bars absolutemaintains the temperature at 197.4° C. Measuring pressure is preferableto measuring temperature because a pressure transducer responds rapidlyto pressure changes while a temperature transducer such as athermocouple is both slow and reflects only the temperature of thematerial imediately adjacent the transducer.

It is known that the reaction rate increases as the size of theparticles of silicon dioxide decreases. Preferably in accordance withthis invention the specific surfaces of the raw materials of calciumhydroxide and silicon dioxide are 18 to 22 square meters per gram whilethe distribution of particle size of both calcium hydroxide and silicondioxide is such that 90 percent of the particles are smaller than onemicrometer in diameter. The specific surface areas given are based uponthe well known nitrogen absorption method. Lower specific surfaces ofreaction products increase the reaction time whereas higher specificsurfaces reduce the reaction time but increase the cost of the rawmaterials (working with gels is more costly than working with solidconstituents of small particle size). Preferably the solid materialcontent of the autoclave is in the range of one to seven percent of thewater in the autoclave (six percent is preferable). Increasing thesolids contents of the reaction products would increase both thereaction time and viscosity of the reactants, substantially increasingthe discharge time of the autoclave.

Thus in accordance with this invention, when the raw material contains72.4% of calcium oxide in calcium hydroxide of 95% purity and silicondioxide of 94% purity, the weight amounts of calcium hydroxide andsilicon dioxide can be calculated as a function of the necessary amountof water (denoted by "X") in the autoclave by the following equations inwhich n is the sum of the weights of calcium hydroxide and silicondioxide expressed in percent of the total weight X of the water inautoclave 10 (i.e., 0<n≦7%):

    1.2nX/220=weight of Ca(OH).sub.2 in kilograms (1)

    nX/220=weight of SiO.sub.2 in kilograms (2)

    X=weight of water in kilograms (3)

For a stochiometric reaction, the weights of the reaction constituentsin kilograms should be selected in accordance with the above equations.Impurities in water such as iron, chlorides, carbon dioxide, potassiumand sodium should be kept to a minimum. Preferably these impuritiesshould be no greater than 1.6 parts per million ("ppm") of calcium, 0.06ppm of iron, 0.15 ppm of magnesium, 0.37 ppm of potassium and notraceable amounts of chloride, sodium and carbon dioxide on a parts permillion basis.

Upon completion of the reaction, the reaction products are transferredfrom the autoclave 10 to holding and mixing vessel 12. The reactionproducts are passed through valve 17 and flow passages 11c, 11d, and 12cduring this transfer as well as through heat exchanger 11. Heatexchanger 11 contains a material, typically water, which will be addedto the next batch of reaction constituents to be placed in the autoclave10.

The contents of autoclave 10 are transferred under pressure fromautoclave 10 to vessel 12. The relative pressures of these twocontainers are controlled by pressure control circuitry 13 (shown inmore detail in FIG. 2). In accordance with my invention, the pressure invessel 12 is held beneath the pressure in autoclave 10 in a controlledmanner (for example, by a constant amount) such that the reactionproducts flow from autoclave 10 to vessel 12 at a rate such that thecrystals in the reaction product are not fractured or broken down. Inone embodiment wherein the flow passage from autoclave 10 to vessel 12through heat exchanger 11 was about 10 meters long and had an insidediameter of about 30 millimeters, the pressure difference betweenautoclave 10 and vessel 12 was about 1.5 bars maxiumum and preferablywas 0.4 bars. Heat exchanger 11 removed enough heat from the reactionproducts that the temperature of the reaction products in vessel 12 wasapproximately 87° C.

A motor 12e is attached to vessel 12 to stir by means of an agitator 12fthe reaction products in vessel 12 to insure that the reaction productsin this vessel remain homogeneous and to allow the mixing of fibrousmaterials with these products. The flow of the reaction products fromautoclave 10 to vessel 12 is, in one embodiment, assisted by theinfluence of gravity obtained by placing autoclave 10 above vessel 12 bythe distance h. This placement however is not essential.

During the transfer of reaction products from autoclave 10 to vessel 12the pressure in autoclave 10 drops. Agitator 10c continues to rotate(but at a slower rate) to prevent vortexing of the reaction products inautoclave 10 during the transfer. Control circuitry 13 operates toensure that the pressure in vessel 12 likewise drops a sufficient amountto maintain the desired pressure difference between autoclave 10 andvessel 12. The way in which this is done will be described later. Uponcompletion of the transfer of the reaction products from autoclave 10 tovessel 12, autoclave 10 and vessel 12 are aproximately at atmosphericpressure. If not, air is released from vessel 12 to bring the pressurein vessel 12 to atmospheric pressure.

Fibrous material comprising preferably a mixture of mineral wool fibersof selected lengths substantially uniformly dispersed in water is thenadded through hatch 12b to vessel 12. Agitator 12f rotates slowly tothoroughly mix this mineral wool with the reaction products in vessel12. Upon completion of this mixing which in one embodiment takes aboutfive minutes, wood pulp fibers are then added to the reaction productsin vessel 12 through hatch 12b. These cellulose fibers are firstsubstantially uniformly dispersed through water and then added to vessel12. Agitator 12f again thoroughly mixes the materials in vessel 12 toinsure a substantially uniform dispersion of the fibrous materialthroughout the reaction products. Upon completion of the mixing, valve12d is opened and the slurry contained within vessel 12 passes bygravity through pipe 12g into a pan from which it is picked up on thesurface of rotating drum 14. Drum 14 comprises a rotating vacuumfiltration unit containing a sieve drum which rotates as a cake of thereacted product forms on its outer surface. The moisture in the slurryis drawn out of the slurry by a vacuum drawn on the center of the drumand the solid material cakes on the surface of the drum. This cake isscraped from the drum and transmitted to the filter press 15.

Filter press 15, of a type well known in the art, presses the cake fromdrum 14 into a desired shape such as a slab or a curved segment. Thecake is placed in the bed 15b of the press on a porous material. Thepressure plate 15a (also called a "platen") is then brought down ontothe top of the cake inside the bed 15b. The pressure created by plate15a together with a vacuum drawn on the bottom of the cake forces waterout the bottom of the filter press. This water passes through pipe 15cinto a first hot water storage tank 20. By opening valve 22 and turningon pump 25 this water will be used to partly fill the autoclave 10 atthe start of the next production cycle.

The clean hot water from heat exchanger 11 is stored in hot waterstorage tank 21 and is subsequently transferred to autoclave 10 and/orboiler 16. Two hot water storage tanks 20 and 21 are used because thewater from filter press 15 has a Ph value of about 11 to 12 and thusshould not be used in boiler 16. In one embodiment the water in heatexchanger 11 is recirculated through storage tank 21 by centrifugal pump24. If the total iron, sodium, chlorine, potassium and magnesiumconcentrations in the water in tank 20 are above the minimumspecifications for these impurities, this water will be discarded.

The above described procedure saves both energy and water, thus givingthis process a substantial advantage over prior art processes. The useof hot water from heat exchanger 11 in boiler 16 (which provides steamto autoclave 10) additionally conserves energy and water.

EXAMPLE

The procedure followed to initiate the reaction in autoclave 10 in apilot line constructed to implement the invention was as follows:

Thirty liters of water from heat exchanger 11 were passed into autoclave10. Agitator 10c was then turned on to stir the water at a selectedspeed of about 90 RPMs. Then 10 liters of water with 1,091 grams ofcalcium hydroxide with a purity greater than 95 percent were added toautoclave 10 and stirred by agitator 10c. This was followed by theaddition of 10 liters of water with 909 grams of silica dust (i.e.,SiO₂) with a purity of greater than 94 percent well dispersed therein.During the filling of autoclave 10, valve 10d was opened to release airfrom the autoclave.

The water added to autoclave 10 was demineralized to enable the resultsof each run to be reproduced as exactly as possible. However,demineralization of the water is not necessary in normal productionprovided the characteristics of the water are known and thereby can becompensated for if necessary and further provided that the amounts ofiron, chlorine, potassium, sodium and carbon dioxide are held inaccordance with the above described specifications for these minerals.The impurities in the calcium hydroxide and silicon dioxide shouldpreferably be on the order of the levels specified in the followingtable.

                  TABLE                                                           ______________________________________                                        Impurity levels in Ca(OH).sub.2                                               CaO             72.80%     min                                                Combined H.sub.2 O                                                                            24.73%     min                                                Free moisture   0.18%      max                                                Silica + insolubles                                                                           0.37%      max                                                Organic matter  0.02%      max                                                Ferric oxide fe.sub.2 O.sub.3                                                                 0.06%      max                                                Aluminum oxide  0.03%      max                                                Magnesium oxide 0.15%      max                                                Sodium oxide Na.sub.2 O                                                                       0.03%      max                                                Potassium oxide K.sub.2 O                                                                     0.01%      max                                                Chloride        0.07%      max                                                Sulphate        0.54%      max                                                Carbon dioxide CO.sub.2                                                                       0.69%      max                                                Total loss on ignition                                                                        25.62%     max                                                Calcium hydroxide Ca(OH).sub.2                                                                95.00%     min                                                Density         0.663      gr/cc                                              Sieve                                                                         Retained 100 mesh 0.31                                                        Retained 200 mesh 0.73                                                        Impurity levels in SiO.sub.2                                                  SiO.sub.2       94%        min                                                SiC             0.2-0.7%   max                                                Fe.sub.2 O.sub.3                                                                              0.05-0.15% max                                                TiO.sub.2       0.01-0.02% max                                                Al.sub.2 O.sup.3                                                                              0.1-0.3%   max                                                MgO             0.2-0.8%   max                                                CaO             0.1-0.3%   max                                                Na.sub.2 O      0.3-0.5%   max                                                K.sub.2 O       0.2-0.6%   max                                                M               0.003-0.1% max                                                C               0.002-0.005%                                                                             max                                                Zn              0.005-0.01%                                                                              max                                                Ni              0.001-0.002%                                                                             max                                                S               0.1-0.3%   max                                                C               0.2-1.0%   max                                                P               0.03-0.06% max                                                Loss on ignition (1000° C.)                                                            0.8-1.5%   max                                                Density         2.20-2.25  gr/cm.sup.3                                        Specific surface area                                                                         18.0-22.0  m.sup.2 /gram                                      Particle size percentage                                                                      90%        is smaller than one                                                           micrometer                                         ______________________________________                                    

When the autoclave was filled, valves 10d and 10e were closed andautoclave 10 was heated to 197.4° C. at an absolute pressure of 15 barsfor one hour and fifteen minutes. While the heat was supplied by usingelectric heating elements, the preferred method is to inject steam intoautoclave 10. During the heating process, agitator 10c remained on at 90RPM. The optimum rotational rate of agitator 10c depends upon the sizeof autoclave 10. The pilot autoclave possessed an inner diameter of 40centimeters and its cylindrical height was 40 centimeters. The outsidediameter of the agitator 10c was 30 centimeters. At 90 RPM, autoclave 10yielded a non-viscous reaction product. A lower RPM was found to yield amore viscous reaction product for the same reaction time while a higherRPM was found to be more likely to damage the crystals formed during thereaction process. Of course the optimum rotational speed of agitator 10cmust be determined experimentally for an autoclave 10 of size differentfrom that used in the pilot line.

During the reaction process, the air pressure in vessel 12 was increasedto a pressure equal to or above the pressure in autoclave 10 by openingvalve 16d to allow compressed air from a compressed air source (notshown) to enter vessel 12. Valve 16d was then closed and pressurecontrol circuitry 13 was turned on. Agitator 12f in vessel 12 was alsoturned on. The reaction process continued for one hour fifteen minutes.At this time, transfer of the reaction products from autoclave 10 tovessel 12 was initiated by first opening valve 16 to allow compressedair to flow from vessel 12 through line 12c, valve 16, line 11d, line11c, to poppet valve 17. Then poppet valve 17 was opened to initiate thetransfer of the reaction product from autoclave 10 to vessel 12. As willbe explained later, the pressure in vessel 12 is automatically broughtto the proper level beneath the pressure in autoclave 10 by controlcircuit 13. As the pressure in autoclave 10 started to sink rapidly, thespeed of agitator 10c was lowered to about 50 RPM to reduce thelikelihood of vortexing in autoclave 10. Vortexing would allow the steamand air pressure within autoclave 10 to escape through valve 17 beforeall of the reaction product in autoclave 10 had been transferred intovessel 12.

Upon completion of the transfer, the vessel 12 was at atmosphericpressure and autoclave 10 automatically went to atmospheric pressure.Poppet valve 17 was then closed and an additional five liters of waterwas added to autoclave 10 and stirred by agitator 10c for two minutes toclean the interior of the autoclave. The cleaning water was thentransferred to vessel 12 by opening poppet valve 17 and allowing thiswater, together with any residual reaction product, to flow into vessel12.

Heat exchanger 11 had previously been filled with about 120 liters ofwater by opening valve 19 and closing valve 18. When filled with water,valve 19 was closed. The flow rate of the reaction products through theheat exchanger was selected so that the reaction products in vessel 12had an average temperature between 85° C. and 90° C. and preferablyabout 87° C. Accordingly, the reaction products on the average droppedabout 110° C. in passing through heat exchanger 11. Heat exchanger 11was designed using well known techniques to obtain this 110° C.temperature drop for the selected pressure difference between autoclave10 and vessel 12.

The reaction product (calcium silicate) in vessel 12 was then slowlystirred to insure uniform temperature.

Mineral wool and cellulose fibers were then added to the reactionproducts in vessel 12 in that order. These materials were not added toautoclave 10 because the high temperature and pressure in autoclave 10combined with the high Ph value of about eleven of the reaction productsin the autoclave would destroy the cellulose fibers. Even though mineralwool would not be destroyed, the addition to autoclave 10 of mineralwool would waste energy. Mineral wool fiber was added first as anaqueous solution in water between 80° C. to 90° C. and preferably atabout 87° C. This mineral wool had previously been mixed in a blenderfor about ten (10) minutes to break down the fiber length of the mineralwool and disperse the mineral wool uniformly in the water. Any nodulesin the mineral wool sedimented out at the bottom of the blender and werenot added to the reaction product in vessel 12. The absence of nodulesin the mineral wool fibers advantageously resulted in a substantiallyuniform structure. Preferably the mineral wool is obtained from athree-wheel spinner with a low shot content.

After the mineral wool was added to vessel 12, vessel 12 was stirred byagitator 12f for ten (10) minutes and then wood pulp, which had beenuniformly dispersed through warm water at a temperature between 80° C.and 90° C. and preferably at about 87° C. by mixing in a blender forabout ten (10) minutes, was added to the reaction products. (If the woodpulp is added prior to the mineral wool, the wood pulp thickens thereaction products in vessel 12 and makes it difficult to properly mixthe mineral wool. However, the mineral wool when added first does notincrease the viscosity of the slurry.) The mixture was stirred byagitator 12f in vessel 12 for about twenty (20) minutes at sixty (60)RPM. Typically, mineral wool by weight comprised 2.5 percent of theweight of the calcium hydroxide and silicon dioxide in the autoclave andthe cellulose fibers by weight also comprised 2.5 percent of theoriginal weights of the calcium hydroxide and silicon dioxide. Themineral wool and cellulose fibers gave mechanical strength to thereaction products when they were formed into slabs or molded products.

The wood pulp preferably was derived from unbleached Kraft paper whichwas flash dried on a sulfite basis. (Such paper is obtainable fromObbala in Finland.) The length of the wood fiber was not critical.During this mixing of the mineral wool and cellulose fibers in vessel12, vessel 12 was at atmospheric pressure.

The reaction products in vessel 12 were removed from this vessel byopening valve 12d and pouring these products through pipe 12g onto a panfrom which they were picked up by the surface of filtration drum 14.Filtration drum 14 preferably comprises a vacuum drum containing a metalscreen of the type made by Sala in Sweden. A vacuum was drawn on thecenter of the drum to yield a pressure of about 0.22 bar and moisture inthe cake formed on the outside face of the drum was drawn into thecenter of the drum and discharged. The cake was scraped off the drum andthen passed to filter press 15 where additional moisture was removedunder vacuum from the reaction products while the reaction products weremolded into a desired shape. Typically, the reaction products(consisting of calcium silicate and more particularly xonotlite) flowingfrom vessel 12 were about six percent by weight solid. The cake on thesurface of drum 14 had a moisture content controlled by the vacuum, thedrum speed, and the cake temperature. The moisture content of the cakescraped from the drum was about 90 percent by weight at the scraper. Thetemperature of the cake coming off the drum was about 40° C. to about50° C. By increasing (decreasing) the vacuum in the drum and slowing(increasing) the speed of the drum, the moisture content in the cake canbe decreased (increased) as desired.

The cake in the filter press was then subjected to a vacuum on thebottom of the filter press to remove additional moisture. Typically,about 22 inches of mercury vacuum was drawn on the bottom of the filterpress. The top surface of the cake in the filter press became dry andthe platen 15a was then brought down on the top of this cake. At themoment the platen 15a touched the top of the cake, no additionalpressure was applied for about 15 seconds. This allowed the vacuum todrain water from the cake while partially sealing the top of the cakewith the platen. Then hydraulic pressure was applied to the platen for afew seconds to bring the pressure on the cake from zero to two bars inabout 15 seconds. Two bars pressure was then applied for 30 seconds.Then in a step function the pressure was increased to about five barsfor one minute. An additional step increase of pressure to 14 bars forthree minutes was applied to the cake in the filter press and then thepressure was reduced to zero with the platen left on the resulting slab.The box surrounding the slab was then moved up at the moment thepressure reached zero and then the platen was released from the pressedslab. The slab was then removed from the filter press and the abovesequence of steps was begun again.

END EXAMPLE

In an alternative and a preferred embodiment, the slurry is takendirectly from vessel 12 to press 15 equipped with both top and bottomsuction whereby the top suction setting is slightly higher than thebottom suction setting, (i.e., gravity is taken into consideration) toensure uniform particle size distribution of the newly formed xonotliteagglomerates in the final product. This prevents unwanted curvature ofthe final product due to larger xonotlite particles settling to thebottom of the filter press in the slurry prior to the removal of waterfrom, and the pressing of, the slurry. Total cycle time is substantiallyreduced by eliminating the rotating drum and delivering the slurry fromvessel 12 directly into press 15. The press is electronically controlledusing well known programming techniques to press the slurry into a slabby gradually stepping up the pressure in accordance with a preprogrammedsequence. Initially a pressure of about two (2) bars is applied forabout thirty (30) seconds (this time can be varied if desired) and thena pressure of five (5) bars is applied for about one and a half (1.5)minutes. This is followed by a pressure of fourteen (14) bars foranother two (2) minutes.

As a feature of this embodiment of the invention, the pressed productcreated by filter press 15 is removed from the filter press by applyinga pressure to the top platen of the filter press so as to generate aslight positive pressure on the top surface of the product formed in thefilter press just beneath the platen. As a result of this positivepressure, the product is gently removed from contact with the adjacentsurface of the platen. The line through which the pressure is applied isnot shown in the drawing but is the same line as is used to pull avacuum on the top surface of the product for the removal of water.

To insure uniform quality product from autoclave 10, valve 17 is apoppet valve (of well known design) placed at the bottom of autoclave 10so that the top surface of the poppet valve is flush with the bottominside surface of autoclave 10. This insures that all reactionconstituents in autoclave 10 are uniformly mixed and participatesubstantially equally in the reaction. To remove the reaction productfrom autoclave 10, the poppet valve 17 is driven upward into autoclave10 to allow the reaction product in autoclave 10 to flow out of theautoclave through valve 17. Advantageously, poppet valve 17 allows thereaction product to flow cleanly and completely from autoclave 10.

To insure uniform quality of the reaction product, the raw material(calcium hydroxide and silicon dioxide) is kept in airtight sealed drumsto prevent them from picking up impurities from the open air. It isparticularly detrimental for the calcium hydroxide to pick up carbondioxide because carbon dioxide distorts the crystalline structure of thecalcium silicate end product.

It is well known that xonotlite converts to beta wollastonite at 865° C.This conversion results in the reaction product converting from Ca₆ Si₆O₁₇ (OH)₂ to CaSiO₃ and H₂ O (water). The water is dissipated from thestructure. The removal of the bound water from the xonotlite crystalduring the conversion process has very little effect on porosity butdoes result in dimensional stability when subsequently the product isbrought up to a temperature in excess of 865° C. The heating of theinsulation formed in accordance with this invention to 865° C. oxidizesthe cellulose fibers in the insulation. These fibers actually oxidize atbetween 300° C. and 400° C. The result is an increase in the porosity ofthe calcium silicate insulation product from about 84% to 89% whilesimultaneously the permeability to gas of the insulation remains atabout 0.01%. This low permeability and high porosity gives to thismaterial its unique characteristics as an extremely efficient hightemperature insulating material. Normally, when the density of aninsulating material is increased, its thermal conductivity alsoincreases. Trials with the material produced by the process of thisinvention, however, have shown that as its density increases, thethermal conductivity of the material goes up far less than wouldotherwise be expected based upon experience with prior art materials.Density depends on slurry quantity and press pressure.

As an added feature of this invention, the autoclave 10 and vessel 12are made of stainless steel to reduce down time necessary to maintainand clean this equipment.

FIG. 2 illustrates the electronic circuit used to control the pressuredifference between autoclave 10 (FIG. 1) and antipressure vessel 12(also called the holding tank) during the transfer operation.

This circuit operates to control a magnetic pressure relief valve 101 onholding vessel 12 so as to maintain the pressure in vessel 12 a selectedamount beneath the pressure in autoclave 10. The pressures in autoclave10 and vessel 12 are sensed by potentiometers R7 and R8, respectivelycoupled to the manometers 10a and 12a attached to autoclave 10 andvessel 12, (FIG. 1) respectively.

Potentiometer R7 provides an increasing resistance with increasingpressure in autoclave 10. Potentiometer R8 provides a decreasingresistance with increasing pressure in vessel 12. Together withresistances R1 and R5 and zener diode D1, potentiometers R7 and R8 forma series connected current flow path between the voltage source +V(varying from 22 to 26 volts DC) and ground. Connected to node 104between resistor R1 and zener diode D1 is one lead of a resistor R4, theother lead of which is connected both to a capacitor C1 and the positiveor non-inverting input lead to operational amplifier A1. The invertingor negative input lead to amplifier A1 is connected to the other lead ofcapacitor C1 and one lead of resistor R3. The other lead of resistor R3is connected to node 105 formed by resistor R2 and zener diode D2. Theother lead of the zener diode D2 is connected to resistor R6 which inturn is connected in series with the parallel connected resistors R9,R10, R11 (R11 is a variable resistor), variable resistor R12, and theparallel connected resistors R13 and R14. Resistors R2, R6 and R12,together with zener diode D2, and the two groups of parallel connectedresistors (R9, R10 and R11; and R13 and R14) form a second resistivedivider network series connected between the positive voltage source +Vand ground. Zener diodes D1 and D2 break down at about 2.4 volts.

The two voltage divider networks are arranged so that in the case wherethe pressure difference between autoclave 10 and vessel 12 is at aselected value (in one embodiment 0.4 bar), the input voltages on theinverting and non-inverting input leads of operational amplifier A1 aresuch that amplifier A1 produces a positive output signal of about 22volts. The input voltage on the non-inverting input lead of amplifier A1is determined by the setting of potentiometers R7 and R8 which arecontrolled by the settings or indications of the two manometers. Thesetwo potentiometers provide in the divider network between +V and grounda sum of resistances corresponding to the nominal difference is pressuredesired between autoclave 10 and vessel 12. The degree of exactness ofthis correspondence depends upon the tolerances of the electroniccomponents. When operated in accordance with the principles of thisinvention, autoclave 10 will be at a higher pressure than vessel 12 inorder to initiate and sustain the transfer of the contents of autoclave10 to vessel 12.

To keep this pressure difference between chosen limits, resistor R11 isset at about 45 ohms and resistor R12 is set in many cases--at about 3ohms for the chosen nominal pressure difference of 0.4 bars. The effectof differences between nominal and actual values of the components inthe circuit of FIG. 2 due to tolerances on component values is takeninto account by empirically adjusting R11. Resistor R11 provides a rangeof coarse adjustment over 100 ohms.

The proper setting of resistor R11 enables adjustable resistor R12 toprovide fine tuning within the desired range, i.e., the range of R11 isgreater than the range of R12 to thereby provide respectively coarse andfine regulation of the desired pressure difference between autoclave 10and vessel 12.

Neglecting component tolerances, R11 should be set in such a way that byadjusting R12 the resistance of the network consisting of R9, R10, R11,R12, R13, and R14 can be varied between 215 and 225 ohms to regulate thepressure difference between zero and 1.2 bars by varying the setting ofR12 only. This ensures that the pressure in vessel 12 is beneath thepressure in autoclave 10 by a selected amount between zero to 1.2 bars.For the component values given infra, in the case when R12 is set at 3ohms, this pressure difference is kept at about 0.4 bars during thetransfer of the contents from autoclave 10 to vessel 12.

When the pressure difference between autoclave 10 and vessel 12 issufficiently large (greater than 0.4 bars for the nominal case) thecurrent through resistor R1 creates a voltage on the positive input leadof amplifier A1 larger in magnitude than the voltage on the negativelead of amplifier A1. The result is that amplifier A1 produces apositive output voltage. This positive output voltage is transmittedthrough resistor R16, diode D4, and resistor R17 to the negative orinverting input lead of operation amplifier A2. The output signal ofamplifier A2 is thus negative relative to the output signal ofoperational amplifier A1 by about 20 volts, thereby turning offtransistor Q1. Accordingly, no significant current passes through Q1 andthe voltage on the emitter of Q1 is almost at ground level.

The positive output signal from amplifier A1 charges capacitor C2 to aselected voltage and produces a voltage drop across resistor R18 andvariable resistor R19. The PC network comprising capacitor C2 andresistors R18 and R19 provides a time delay of about one second which,as will be seen shortly, decreases the oscillation frequency of thetotal system.

During the operation of the system, as the pressure in vessel 12increases as a result of the contents of autoclave 10 transferring intovessel 12, the resistance provided by potentiometer R8 decreases.Likewise, as the contents of autoclave 10 transfer out of autoclave 10into vessel 12, the pressure in autoclave 10 drops thereby decreasingthe resistance of potentiometer R7.

This simultaneous decrease in the resistances of both potentiometers R7and R8 increases the current passing through resistor R1 therebylowering the voltage applied to the positive input lead of amplifier A1.As this voltage drops beneath the voltage on node 105 applied to thenegative input lead of amplifier A1 (the voltage on node 105, in turn,is controlled by the settings of variable resistors R11 and R12) theoutput signal from amplifier A1 switches from a high level positiveoutput signal of about 22 volts to a low level output signal with apotential slightly above ground. Light emitting diode D3 stops emittinglight when the output signal from amplifier A1 switches from a high tolow level. Diode D3 together with diode D5 (also a light emitting diode)and the two manometers serve to allow an operator to visually check thefunctioning of the system. Diode D4 prevents current from flowing fromcapacitor C2 back through resistor R16 to the output lead of amplifierA1 and thus prevents the activation of diode D3. Capacitor C2, however,discharges through resistors R18 and R19. The discharge time ofcapacitor C2 is determined by the one second RC time constant of thecombination of capacitor C2 with the resistors R18 and R19 (this timeconstant can be varied as desired). As the voltage on capacitor C2drops, so does the voltage on the negative input lead to amplifier A2.When this voltage drops beneath approximately +8 volts, the outputsignal from amplifier A2 switches from low level (slightly above ground)to a high level positive output signal thereby turning on and saturatingpreviously switched off transistor Q1. Current then flows throughtransistor Q1 creating a current through the winding of relay switch S1which turns on the electrically actuated valve 101. When open,electrically actuated valve 101 on holding vessel 12 releases gas from,thereby lowering the pressure in, this vessel. Sufficient pressure isreleased from vessel 12 to increase the resistance of R8 sufficiently tocompensate for the decrease in resistance R7 due to the decrease inpressure in autoclave 10. As soon as the total series resistance ofpotentiometers R7 and R8 increases in value to raise the voltage on node104 above the voltage on node 105, the output signal from amplifier A1swings to its high level thereby driving the output voltage fromamplifier A2 to a low level and thus turning off transistor Q1. Thisshuts pressure relief valve 101 on anti-pressure vessel 12 therebypreventing the pressure in this vessel from dropping beneath thepressure in autoclave 10 by more than the selected amount. The operationof this circuit continues to control the pressure difference betweenautoclave 10 and vessel 12 in the above manner, thereby allowing thetransfer of the contents of autoclave 10 to vessel 12 in a controlledmanner.

The existence of oscillation in the system during the transfer of thecontents of autoclave 10 to anti-pressure vessel 12 can be seen byconsidering FIG. 3 in conjunction with FIG. 1. Assuming initially thattransistor Q1 is off and valve 101 is thus closed, the pressure inautoclave 10 is above the pressure in vessel 12 by a selected amountdetermined by the settings of variable resistors R11 and R12 (mainlyR12) shown in FIG. 2. As stated above, typically this pressuredifference is about 0.4 bars. As the transfer proceeds, the pressure inautoclave 10 drops due to the departure of its contents and the pressurein vessel 12 rises due to the arrival of the reaction products in vessel12. Consequently, the pressure difference (ΔP=P₁₀ -P₁₂ where P₁₀ is thepressure in autoclave 10 and P₁₂ is the pressure in vessel 12) betweenthese two vessels drops as shown by the downward pointing arrow in the"ΔP" column of step a of FIG. 3. Pressure control unit 13 (FIGS. 1 and2) senses this drop and turns on transistor Q1 thereby opening valve 101to release pressure from vessel 12. As a result, the pressure in vessel12 drops as indicated by the downward pointing arrow in the columnlabeled " Pressure in vessel 12" in step b of FIG. 3. The pressure inautoclave 10 continues to drop as indicated by the downward pointingarrow in the column labeled "Pressure in Autoclave 10" in FIG. 3, andthe pressure difference ΔP between these two vessels increases as shownby the upward pointing arrow in the "ΔP" column of FIG. 3.

When the pressure difference ΔP between autoclave 10 and vessel 12exceeds a selected magnitude, pressure control circuitry 13 detects thisincrease in the pressure difference and closes valve 101 therebyallowing the pressure in vessel 12 to build up again as the contents ofautoclave 10 continue to transfer into vessel 12. Consequently, as shownin step c of FIG. 3, the pressure in autoclave 10 continues to drop, thepressure in vessel 12 increases and the pressure difference ΔP betweenthese two vessels begins to decrease again. As the transfer continues,control unit 13 senses a decrease in the pressure difference between thetwo vessels beneath a selected amount, opens valve 101 thereby againlowering the pressure in vessel 12 and allowing the pressure differenceΔP to again increase. This cycling continues at a rate which is afunction of the RC time constant of the circuit comprising capacitor C2and resistors R18 and R19 shown in FIG. 2, the size of the orifice inrelease valve 101, and the flow rate of the reaction products fromautoclave 10 to vessel 12.

From the above description, it is seen that the control system providedby the circuit of FIG. 2 operates in a binary fashion; that is, thepressure relief valve 101 on vessel 12 is either open, thereby releasingpressure from this vessel, or closed, thereby allowing pressure to buildup in this vessel. Feedback between the two manometers R7 and R8attached to the autoclave 10 and anti-pressure vessel 12 respectively isprovided by the pressure in each of these two tanks. As the contentstransfer from the autoclave 10 to the anti-pressure vessel 12, thepressure in autoclave 10 drops and the pressure in anti-pressure vessel12 rises. Opening valve 101 releases gas from vessel 12 thereby allowingthe difference in pressure between autoclave 10 and vessel 12 to rise.Closing valve 101 stops gas from being released from vessel 12 therebyallowing the pressure to build up in this vessel during the transfer ofcontents from autoclave 10 to vessel 12.

If desired, one can insure that the pressure in vessel 12 falls morerapidly when valve 101 is opened than it builds up due to the arrival ofthe contents from autoclave 10 by appropriately selecting the minimumorifice of valve 101 to allow the drop in pressure in vessel 12 to occurmore rapidly than the build up of pressure in this vessel. In thelimiting case, valve 101 will remain open during the whole transferoperation. In most situations, however, valve 101 is sized so that itwill cycle open and closed several times during the transfer operation.

In an alternative embodiment, the electronic control signal can bereplaced by a throttle valve or a valve and a vent pipe. At the start ofthe transfer operation, a suitable pressure difference can beestablished between autoclave 10 and pressure vessel 12. To start thetransfer, the valve between autoclave 10 and vessel 12 is opened andsimultaneously or subsequently, as desired, the pressure release valveon the top of vessel 12 is opened and left open during the transferprocess. As a result, the reaction product from autoclave 10 flows intovessel 12 at an instantaneous rate determined by the instantaneouspressure difference between autoclave 10 and vessel 12. This pressuredifference is controlled by the size of a vent pipe or the setting ofthe throttle valve. This embodiment avoids the use of a control circuitbut has the potential disadvantage that the transfer is not as preciselycontrolled.

The transfer operation represents a balance of competing interests. Tominimize the unwanted continuation of the reaction of the constituentsin autoclave 10, the contents of autoclave 10 should be transferred tovessel 12 in a known but short time. Typically a time of ten to twelveminutes has been found practical both to minimize the continued reactionof these constituents and to increase the efficiency of the system byminimizing cycle time. Of course, longer transfer times can be designedinto the system if desired. On the other hand, too rapid a transfer ofthe contents from autoclave 10 to vessel 12 results in a fracturing anda rupturing of the calcium silicate crystals. Accordingly, I havediscovered that one can balance these two competing interests to yield atransfer rate which both decreases the total time necessary to process abatch of calcium silicate from the start of the reaction to transfer ofthe reaction product from autoclave 10 to vessel 12 and at the same timeprevent the rupture or disassociation of the reaction product. Thus inaccordance with my invention, the process is optimized to obtainefficient utilization of the equipment together with a reaction productwhich has predictable desired characteristics.

The structure described above allows the transfer of the contents ofautoclave 10 to vessel 12 under a substantially constant pressuredifference even as the pressure in autoclave 10 drops, thereby ensuringthat the flow of the reaction products from autoclave 10 through theheat exchanger 11 occurs at a substantially controlled and constant flowrate such that the reaction products are not fractured or degenerated ina manner detrimental to the final characteristics of the reactionproducts. The length of the drain system comprising valve 17, pipes 11cand 11d, valve 16 and pipe 12c in FIG. 1 and the pressure difference issuch that the flow rate of the reaction product during the discharge isselected to avoid degeneration of this product. The use of the heatexchanger to remove heat from the reaction product assists instabilizing this product in a rapid manner during its flow fromautoclave 10 to vessel 12 so that the final product made using thereaction product has a uniformity of characteristics previouslyunattainable. In one embodiment this is obtained by insuring that theReynolds number of the flow products is substantially laminar (below theturbulent flow conditions which create unwanted shear forces on thereaction product). The Reynolds number of the flow products is beneath4,000 and preferably about 200. However Reynolds numbers in the range of200 to 600 have been employed with satisfactory results.

Comparative data of both Newtherm 950, a prior art calcium silicateinsulation produced by Turner and Newalls in England, and the productmade by the process of my invention (denoted Wictherm 1000) are givenbelow.

    ______________________________________                                        20° C.                                                                             400° C.                                                                        600° C.                                                                        800° C.                                                                      900° C.                                                                      1000° C.                       ______________________________________                                        Flexural                                                                      strength in                                                                   bars*                                                                         Newtherm                                                                      950     5.5     6.1     6.8   7.2   --    7.4                                 Wictherm                                                                      1000    20.1    22.5    29.4  22.5  --    16.7                                Com-                                                                          pressive                                                                      strength in                                                                   bars**                                                                        Newtherm                                                                      950     1.5     ***     ***   ***   --    ***                                 Wictherm                                                                      1000    9.7     10.9    2.9   2.6   --    2.0                                 Shrinkage                                                                     in %                                                                          Newtherm                                                                      950**   --       0.44    1.15 --    --     8.25                               Wictherm                                                                      1000*   --       0.87    1.07  1.77 1.82   3.05                               Corner-                                                                       strength in                                                                   bars**                                                                        Newtherm                                                                      950     34.3    --      --    --    --    --                                  Wictherm                                                                      1000*   210     --      --    --    --    --                                  ______________________________________                                         *Measurements were obtained from samples at indicated temperatures.           **The measurements were obtained from samples that were heated to             indicated temperatures and subsequently cooled down to room temperature.      ***Not measurable, values smaller than 0.7 bar.                          

Also determined for Wictherm 1000 were porosity and K-factors (K is theheat transfer coefficient) with following results:

    ______________________________________                                        Temperature in °C.                                                                  Porosity of Wictherm                                             ______________________________________                                         20° 84.0%                                                             1000°                                                                              89.4%                                                             ______________________________________                                                    K-value of Wictherm in K cal per hour, per                                    meter degree centigrade (K cal hr.sup.-1 m.sup.-1                 Temperature in °C.                                                                 °C..sup.-1)                                                ______________________________________                                         20° 0.07                                                               300°                                                                              0.11                                                               600°                                                                              0.17                                                               900°                                                                              0.22                                                              1000°                                                                              0.24                                                              ______________________________________                                    

K-values were determined using the hot-wire method. Other K-values forWictherm were obtained using the hot/cold face method and expressed inmean temperatures. Also shrinkages in length, width and thickness weredetermined, giving the following results:

    ______________________________________                                                  k                        %                                          t in °C. (mean)*                                                                 Kcal/m/h/°C.                                                                       t in °C.                                                                            Shrinkage                                  ______________________________________                                        200°                                                                             0.0555                   2.22% l                                    400°                                                                             0.0758      Soaked at 1000°**                                                                   2.19% w                                    600°                                                                             0.1055      for 12 hours 4.88% th                                   800°                                                                             0.1344                                                              ______________________________________                                         *The test was carried out according BS 1902 Part 1A, 1966                     **According BS 2972, 1975.***                                                 ***BS means "British Standard                                            

The values of the components shown in FIG. 2 used in the preferredembodiment of this invention are as follows:

    ______________________________________                                        R1   1KΩ       C1     0.1 μF. ± 10%                                                                      160V                                   R2   1KΩ       C2     47 μF.  40V                                    R3   4.7KΩ     C3     4.7 μF.                                                                               63V                                    R4   4.7KΩ     C4     0.1 μF. ± 10%                                                                      400V                                   R5   100Ω      C5     0.1 μF.                                                                              1000V                                   R6   100Ω      C6     0.1 μF.                                                                              1000V                                   R7   Potentiometer 215 Ohm                                                    R8   Potentiometer 215 Ohm                                                                         D1     Zenner 2.4V                                       R9   100Ω      D2     Zenner 2.4V                                       R10  100Ω      D3     LED (Red)                                         R11  Variable 100Ω                                                                           D4     Signal Diode 1N4148                               R12  Variable 10Ω     (Equiv. 1N914)                                    R13  1KΩ       D5     LED (Green)                                       R14  220Ω                                                               R15  4.7KΩ     A1     μA741(μA/LM741CN)                           R16  2.2KΩ     A2     μA741(CA741CE, RCA)                            R17  2.2KΩ                                                              R18  10KΩ      Q1     R52041                                            R19  Variable 100KΩ                                                     R20  4.7KΩ     +V     24 volts                                                                      Varying from 22 to                                                            26V DC                                            R21  10KΩ                                                               R22  220KΩ     101    Electrically actuated relief                                                  valve (Herion, Germany)                                                       or any other type.                                R23  470KΩ                                                              R24  2.2KΩ     102    Voltage source 330V AC                                                        50 Hz                                             R25  22KΩ             fuses 500V, 25A                                   R26  10KΩ      103    Fuse 250V, 0.5A                                   ______________________________________                                    

All resistance values have a tolerance of ±5% Tolerances of C₂, C₃, C₅,C₆ and zener diodes are not critical.

While this invention has been described as operating with asubstantially constant pressure difference between the pressure invessel 12 and autoclave 10, it should be recognized that this pressuredifference, can, if desired, be varied in accordance with a prearrangedschedule, as a function of time.

It should also be recognized that the water from heat exchanger 11 can,if desired, be used for purposes other than a reaction constituent inthe next batch of calcium silicate to be formed in autoclave 10. Forexample this water can be used as a source of heat or in other types ofreactions.

As a feature of this invention, the density of the resulting calciumsilicate insulation product can be varied by varying the pressureapplied to a given amount of calcium silicate in filter press 15. Thefinal density is determined by the filter press pressure and by theamount of calcium silicate in the slurry placed in filter press 15.

The final insulation product after drying typically contains betweenthree to five percent by weight of water. It is not economic to furtherdry the calcium silicate insulation because it will merely absorb waterfrom the atmosphere.

The above description is exemplary only and those skilled in the artwill be able to implement other embodiments of the invention of thisdisclosure without departing from the claimed invention.

I claim:
 1. A system for forming a calcium silicate reaction productcomprising:an autoclave for receiving calcium hydroxide, silicon dioxideand water and for reacting these constituents to form a calcium silicateslurry; a holding vessel for receiving the calcium silicate slurry fromsaid autoclave; means for mixing fibrous material with said calciumsilicate slurry in said holding vessel; a flow passage connecting saidautoclave to said holding vessel for allowing the passage of said slurryfrom said autoclave to said holding vessel; means for transferring heatfrom said slurry during its passage through said flow passage; and meansfor maintaining the pressure in said holding vessel a controlled amountbeneath the pressure in said autoclave such that during the transfer ofthe slurry from said autoclave to said holding vessel there will beminimal structural damage to the slurry during its transfer to saidholding vessel.
 2. A system as in claim 1 wherein said flow passage forallowing the passage of said slurry from said autoclave to said holdingvessel and said means for transferring heat comprise a flow passage witha heat exchanger formed adjacent to at least a portion of said flowpassage thereby to allow a portion of the heat in said slurry totransfer to another medium.
 3. A system as in claims 1 or 2 wherein saidmeans for controlling the pressure in said holding vessel to acontrolled amount beneath the pressure in said autoclave comprises:valvemeans mounted on said holding vessel; a first pressure transducerpositioned and arranged to measure the pressure in said holding vesseland produce a first signal representative thereof; a second pressuretransducer positioned and arranged to measure the pressure in saidautoclave and produce a second signal representative thereof; and meansresponsive to said first signal and said second signal for controllingthe position of said valve means on said holding vessel thereby tomaintain the pressure in said holding vessel substantially at saidcontrolled amount beneath the pressure in said autoclave.
 4. A systemfor forming a reaction product comprising:an autoclave for use informing said reaction product, a holding vessel for receiving thereaction product formed in said autoclave; a flow passage connectingsaid autoclave to said holding vessel, means for maintaining thepressure in said holding vessel in a controlled manner beneath thepressure in said autoclave during the transfer of the reaction productin said autoclave through said flow passage to said holding vessel; andmeans for removing a portion of the heat from said reaction productduring the passage of said reaction product through said flow passage.5. The system as in claim 4 wherein said means for maintainingcomprises:means for detecting the pressure in said holding vessel andproducing a first output signal representative thereof; means fordetecting the pressure in said autoclave and producing a second outputsignal representative thereof; and means responsive to said first outputsignal and said second output signal for holding the pressure in saidholding vessel a substantially constant amount beneath the pressure insaid autoclave.
 6. The system as in claim 4 wherein said means forremoving a portion of the heat from said reaction product furthercomprises:means directly adjacent at least a portion of said flowpassage for transferring a portion of the heat from the reaction productto a heat storage medium.
 7. A system comprising:an autoclave for use informing a reaction product; a holding vessel for receiving said reactionproduct; a pressure release valve on said holding vessel; a flow passageconnecting said autoclave to said holding vessel; and means forcontrolling the pressure in said holding vessel to be no more than afirst selected amount and no less than a second selected amount beneaththe pressure in said autoclave.
 8. A system as in claim 7 wherein saidmeans for controlling comprises:means for detecting the pressure in saidholding vessel and producing a first output signal representativethereof; means for detecting the pressure in said autoclave andproducing a second output signal representative thereof; and meansresponsive to said first output signal and said second output signal foropening said valve to reduce the pressure in said holding vessel whenthe pressure in said holding vessel is less than said second selectedamount beneath the pressure in said autoclave and for closing said valvewhen the pressure in said holding vessel is greater than said firstselected amount beneath the pressure in said autoclave.
 9. A system asin claim 8 wherein said means responsive to said first output signal andsaid second output signal comprisesan electrical circuit for producingan output signal when the pressure in said holding vessel is less thansaid second selected amount beneath the pressure in said autoclave andfor producing no output signal when the pressure in said holding vesselis greater than said first selected amount beneath the pressure in saidautoclave; means connected to open and close said valve; and means forapplying said output signal from said electrical circuit to said meansconnected to open and close said valve thereby to open said valve inresponse to said output signal and to close said valve during theabsence of said output signal.